1. Field of the Invention
The present invention is in the field of electrocardiography and more specifically relates to a system for the ambulatory monitoring of pacemaker operation and the subsequent analysis of pacemaker malfunction.
2. The Prior Art
The present invention is another step forward in the continuing development of instrumentation for ambulatory (Holter) monitoring. Such techniques are more widely used each year owing to a growing appreciation of the diagnostic value of measurements made while the patient pursues his daily activities. Typically, the measurements are recorded on a portable tape recorder affixed to the patient. The recorded tape may subsequently be scanned, analyzed, and plotted. A 24-hour recording can be scanned and analyzed in less than half an hour, and sporadic pauses, trachycardia or bradycardia, and premature contractions can be detected with confidence. It is also possible to detect and to count premature beats, either ventricular or supraventricular.
An early example of a system for recording and playing back ECG signals is found in U.S. Pat. No. 3,215,136, issued Nov. 2, 1965 to Holter, et al. The present state of the art is represented by the device disclosed in U.S. Pat. No. 4,006,737, issued Feb. 8, 1977, to Cherry and in U.S. Pat. No. 4,073,011, issued Feb. 7, 1978, to Cherry and Anderson for "Electrocardiographic Computer". A reissue application of the latter patent was filed Apr. 24, 1978, and a divisional application Ser. No. 773,618, filed Mar. 2, 1977, is directed to a recorder for cardiac signals. A much improved recorder is described in application Ser. No. 918,698 filed June 23, 1978, for "Recorder for Ambulatory Monitoring of Electrocardial Signals" by Shu and Squires.
At the same time that the aforementioned ambulatory monitoring techniques were being developed, other technologists were working to make practical the use of artificial pacemakers, implanted in patients to stabilize their heart rates. Pacemakers are not routinely implanted and there are presently more than 50,000 people in the United States dependant on implanted pacemakers.
Artificial pacemakers supply an electrical stimulation to the heart when it tends to beat irregularly due to a partial or complete block in the heart's conduction system. There are two main types of pacemakers: the fixed rate pacemaker and the demand pacemaker. As the names indicate, the fixed rate pacemaker attempts to pace the heart 100% of the time at a fixed frequency. It soon was recognized that the fixed rate pacemaker was not desireable in those situations in which a partial block of the heart's conduction system left the heart capable of spontaneously beating for appreciable times. In this situation, competition between the fixed rate pacemaker and the remaining spontaneous heart action led to problems. It was this situation which led to the development of the demand pacemaker. The demand pacemaker senses the heart's spontaneous action and operates only when the spontaneous action is inadequate.
Although the reliability of pacemakers is generally excellent, three major types of pacemaker failure have been documented: lead or electrode failure, circuit failure, and battery exhaustion, the latter being by far the most frequent. These types of failure may manifest themselves in any of several observable types of abnormal pacemaker operation. One manifestation is abnormal bradycardia, wherein the time interval between successive spontaneous heart beats is longer than the pacemaker should permit without intervention. That is, the pacemaker fails to apply a stimulative pulse even though the spontaneous beats of the heart are excessively slow.
Somewhat the opposite of abnormal bradycardia is the manisfestation of pacemaker failure known as failure-to-sense. In this mode, a spontaneous heart beat is followed prematurely by a pacemaker spike. That is, the pacemaker spike occurs sooner after the spontaneous beat than it should if the pacemaker were functioning properly. This type of failure appears to indicate that the pacemaker failed to sense the spontaneous heartbeat and went ahead to produce a stimulative spike in spite of the spontaneous operation of the heart.
Another manifestation of pacemaker failure is the observed failure-to-capture. This defective mode of operation results when a pacing spike fails to be followed within 30 milliseconds by the onset of a heartbeat. This mode of failure appears to indicate that the spike generated by the pacemaker was inadequate to trigger a heartbeat.
In contrast to the aforementioned three manifestations of pacemaker failure is the normal mode of operation, wherein a pacemaker spike is followed by the onset of a heartbeat within five to forty milliseconds later. Like the paced beat, the fusion beat is also considered to be normal. A fusion beat results when a pacemaker impulse is followed within five milliseconds by the onset of a heartbeat. This indicates that the heart is spontaneously beating at approximately the frequency called for by the pacemaker.
Failure-to-sense, failure-to-capture, and abnormal bradycardia may occur intermittently making detection and confirmation of pacemaker malfunction difficult, in accordance with Murphy's Law. Clearly, the detection and confirmation of intermittent pacemaker malfunction would be greatly facilitated if the operation of the pacemaker were monitored over an extended time period, and under a wide range of patient activities. For this reason, Holter monitoring of pacemaker operation would appear to offer the ideal solution.
However, when one records and scans a tape from a patient with an artificial pacemaker, a number of frustrating difficulities arise if there is alternation between paced and spontaneous beats. It may indeed be difficult or impossible to detect intermittent pacemaker malfunction unless it causes a significant disturbance of heart rate.
There are two principal reasons for this. First, in commerically available Holter monitoring equipment, the frequency response of the recording playback system is usually intentionally tailored by the manufacturer to minimize false signals introduced by muscular contractions and other artifacts. As a result of the attenuation of this specially shaped frequency response, the pacemaker spikes tend to be slurred and greatly reduced in amplitude, making it often impossible to distinguish on the basis of structure a paced beat from a premature ventricular contraction or other wide but spontaneous QRS complex.
Confirmation of pacemaker malfunction difficult. Clearly, the detection and ambulatory monitoring equipment is that the features of pacemaker sensing failure or capture failure are simply not visible on the display of the playback unit when the recorded tape is played back at higher scanning speeds, such as X30 to X120. Even those scanners which have the ability to count premature ventricular contractions are not able to recognize the pacemaker related events.
Thus, there is a need in the art for a system to permit ambulatory monitoring of pacemaker operation over time periods measured in hours, and to permit rapid scanning and analysis of the recorded data so as to recognize the occurence of intermittent pacemaker malfunction.